It is conventional to manufacture solenoidal magnet coils, such as used in imaging systems such as nuclear magnetic resonance (NMR) or magnetic resonance imaging (MRI) by winding wire “in situ” into slots formed in a machined former. It is also known that a required field homogeneity may be more readily achieved by producing end coils of lesser diameter than the diameter of coils located towards the centre of the solenoidal arrangement.
Such arrangements have been achieved by having a former with slots of appropriate position and diameter to locate and retain such coils. However, particularly in the field of superconducting magnets for imaging systems such as nuclear magnetic resonance (NMR) or magnetic resonance imaging (MRI), it is required to reduce the overall length of the solenoidal magnet as far as is possible, to improve access to a human or animal patient, or other object to be imaged, and to reduce claustrophobia of the patient when placed in the imaging system, as well as to reduce the costs of other system components such as cryogen vessel and outer vacuum container, well known in the art. It is further required to reduce, as far as is possible, the length of wire used in winding of the coils. This is primarily due to the high cost of superconductive wire per unit length, but also serves to reduce the weight of the whole system, which in turn reduces the mechanical strength requirement for suspension components, which in turn may be used to reduce the size of the suspension components, reducing thermal influx to the magnet and its cooling system.
A known solution to these requirements involves the use of separately moulded end coils. In such an arrangement, a reduced-length inner former is used, and is not wound with end coils. The inner former is typically an aluminium tube with coils wound into slots formed on its outer surface. The coils are typically resin impregnated in situ. End coils are wound in a separate mould. The end coil thus wound is typically impregnated with a thermosetting resin which is allowed to cure. The moulded coils are then released from the mould to be attached to the end of the inner former as a separate article. Typically, end formers may be provided. These may be accurately-machined mechanical arrangement for supporting the end coils of the inner magnet and outer shield coils. They are attached to the inner former during assembly by any suitably accurate mechanical process, typically by bolting the former parts together. The end coils are typically retained onto the end former by clamp rings. The benefits of such an arrangement include the fact that material of the former is not provided on the inner surface or the end surface of the end coils. The absence of former material on the inner radial surface means that the inner diameter of the end coil may be reduced to the inner diameter of the inner former, reducing the length and so also the cost of superconducting wire required as compared to an arrangement where all coils are wound onto an outer surface of a solid former. The absence of former material on the axial end surface of the end coil means that the overall length of the solenoidal magnet may be reduced by the length dimensions of the former material conventionally provided beyond each axial end of the end coils.
FIG. 1 shows a quarter section of a known arrangement, having coil 20 with an outer crust layer 2, for example, thermosetting resin impregnated with glass fibre or glass beads. This acts as a thermal diffusion barrier. The crust also provides a step feature 3. A structural web 4 is mechanically linked to the former. A thrust ring 5, typically of aluminium alloy, is welded 6 to the web 4. Thrust ring 5 includes an oversleeve which retains the end coil in position. Coil 20 is placed within the thrust ring. Balance shims 9 may be provided to adjust the axial position of the coil, and to compensate for asymmetries between the ends of the magnet. A coil clamp ring 7 bears against the step 3 in crust 2, retaining the coil in position against the thrust ring 5. Fasteners such as bolt 8 press the clamp ring into contact with the step 3. The axis of the solenoidal arrangement is parallel to the line A-A.
The known moulded end coils may suffer from quenches particularly in turns near the coil's inner radius. The field strength is particularly high at the inner radius, possibly double that present at the coil's outer radius.
Moulded coils are known to be relatively inaccurate. This may be caused by differential shrink of the impregnating material during curing. The use of moulded coils mounted onto a former leads to the probability of patchy contact between the coil and the former. In operation, the magnet coils are subjected to very high forces. The patchy contact may cause these forces to deform the coil or the former at points of high local stress. This yielding may cause a quench. The combination of high forces and patchy contact may also combine to cause some circumferential movement of the coil, which again may lead to a quench, or departure of the magnetic field characteristics from the optimal situation. It is accordingly conventional to use shims to ensure a precise tight and accurate fit of the moulded coil to its former.
Balance shims are known in conventional magnet arrangements for adjusting the axial position of outer coils. Typically, resin-impregnated moulded coils are used as end coils. Their axial position may be varied during assembly to improve the bare magnet homogeneity towards the designed specification. Shims of an electrically insulating, non-magnetic material such as Mylar® polyester sheet are placed between the end coil and the former. The number and thickness of the sheets are chosen to adjust the position of the coils to optimise the bare magnet homogeneity. The primary purpose of the axial shims is to correct for error introduced by the use of moulded coils.
Adjacent radial surfaces of the coil and its former may not be circular. The inner diameter of the moulded coil may not be concentric with its outer surface.
It is known to provide an aluminium former oversleeve gripping the coil tightly on its radially outer surface A2 in an attempt to improve quench training behaviour.
However, such arrangements have suffered from certain drawbacks, some of which will now be briefly described.
When the magnet assembly is cooled to operating temperature, which may be as low as 4K, the former, which is typically of aluminium or an aluminium alloy, shrinks in diameter onto the end coil, which is typically primarily of copper. This provides a tight grip of the former onto the coil, intended to retain the coil securely in position. However, this tight grip may in fact be the source of quench events, if the end coil moves during operation—a so-called stick-slip event. The effect of stick-slip events on operation may be reduced by training: repeatedly ramping up and ramping down current in the magnet, so that the coils settle into a stable position.
The moulded end coils have to be wound separately from the rest of the solenoidal magnet, which increases machine set-up time and makes turns balancing more difficult. Turns balancing is the name given to a sequence of steps carried out during the assembly of a magnet, in order to compensate for any deviation of the manufactured magnet from the design, while still achieving approximately the designed magnetic field. The magnet must be assembled with high precision, and take into account the manufacturing tolerances of individual components. Typically, an aluminium former is machined with slots, into which a copper-based superconducting wire is wound.
In one turns balancing sequence, the following steps are performed. The formed dimensions of the slots and the actual cross-section of the wire may combine to mean that the desired number of turns per layer cannot be accommodated, or that a larger number of turns and/or axial shims, will be required to completely fill the axial length of the slot. It is important that each slot be completely filled along its axial length, since any scope for movement may lead to a quench of the magnet in operation. The dimensions of the slots formed in the former are accurately measured, and compared with an accurate measurement of the cross-section to the superconducting wire to be used. The coils are at least partially wound, and then measured for dimensions and number of turns. With these measurements known, a simulation of the finished magnet may be performed to provide a predicted homogeneity. It may be found necessary to vary the number of turns per layer from the design specification in order to completely fill the slot. In this context, attention must be paid to the cross-section of the wire, as this may not be constant along its length, and different numbers of turns may be placed on different layers. Turns can be added or subtracted to/from any of the coils in the magnet to return the predicted magnetic field to its designed bare-magnet homogeneity. It is preferred to add or subtract whole turns only, and it may be necessary to adjust the number of turns on more than one coil to achieve the designed bare-magnet homogeneity without resorting to partial turns. This adjustment of the number of turns on he coils of the magnet in order to achieve a designed magnetic field is known as turns balancing.
If a moulded end coil is used, it cannot take part in turns balancing, but other coils in the magnet need to be adjusted to compensate for any error in the moulded end coils.
Due to tolerance stack-up on the parts of the typical multi-part mould for the end coils, and non-uniform shrinkage of the resin impregnant during gelling, curing and cooling of the end coil, the external cylindrical surface of the finished end coil is typically neither perfectly round, nor concentric with the internal cylindrical surface of the windings of the end coil.
Since the dimensions and the finish of the end coil's surfaces cannot be accurately controlled, it is necessary to use a relatively expensive shimming process to achieve the required tight fit between the end coil and former which retains the remaining coils of the magnet. This is a time-consuming process which is difficult to control. If the fit between the end coils and the former is not sufficiently good, this may lead to quench events. For example, known stick-slip movement of the coil, caused by weak frictional bonds breaking and the coil suddenly moving when in operation can lead to quenches, due to heat generated by friction during the movement of the coil. Similarly, patchy contact of the end coils to the former will produce high local contact stresses, and may deform either or both of the coil and the former. These effects may encourage quench events.
Such drawbacks accumulate such that the end coils are rarely concentric with the rest of the magnet coils, resulting in poor control of transverse homogeneity.
Furthermore, the multi-part mould typically used to form the end coils requires frequent re-coating which adds to the production time and cost.